The Last of Us 2 is currently in production and is coming to PlayStation 4 in 2015, according to a source "within Sony" (translated from Portuguese).
Gaming website Omelete claims to have received this information from their unnamed source. With little else to go on--and knowing that Naughty Dog is currently developing both The Last of Us' Left Behind DLC and the next Uncharted game--it would be wise to take this information with some trepidation.
Of course, it's not unreasonable to think Naughty Dog could release a Last of Us sequel two years after the original's release. That two-year release cycle has defined every main entry in the Uncharted series. The Uncharted 2 team, in particular, took three years to develop The Last of Us after moving to the new franchise.
Naughty Dog is known for producing games of astonishing quality in comparatively short time spans, but the brand-new platform that is PlayStation 4 could introduce wrinkles. Still, with PS4 being much easier to develop for than its predecessor, a shortened development time for The Last of Us 2 isn't unthinkable--especially if the Uncharted 4 team is sharing certain assets.
Regardless, this is all speculation, so drop a comment below and tell us if you think there's anything valuable in this report.